Replaced the name of the global variable holding the input data in the Javascript expression evaluator

[JBBianchi]

The name previously chosen to hold the data passed to a expression, input, was too generic and might easily lead to collision with provided arguments. Therefore, it's been replaced by a less common symbol: $. This basically allows the arguments dictionary to contain an entry with the input key without colliding with the data.

Given the following input data:

{
   "foo": "bar",
   "answer": 42,
}

And the expected output:
The Answer to the Ultimate Question of Life, the Universe, and Everything is 42, and foo = bar

Previously, the expression would have been:
"The Answer to the Ultimate Question of Life, the Universe, and Everything is " + input.answer + ", and foo = " + input.foo

And now is:
"The Answer to the Ultimate Question of Life, the Universe, and Everything is " + $.answer + ", and foo = " + $.foo

Alternatives providing the same output:

• using interpolation: `The Answer to the Ultimate Question of Life, the Universe, and Everything is ${ $.answer }, and foo = ${ $.foo }`
• wrapping the expression in ${}: ${ "The Answer to the Ultimate Question of Life, the Universe, and Everything is " + $.answer + ", and foo = " + $.foo }
• interpolation + wrapping the expression in ${}: ${ `The Answer to the Ultimate Question of Life, the Universe, and Everything is ${ $.answer }, and foo = ${ $.foo }` }

[cdavernas]

LGTM!!!